The Finn, whose retired predecessor and 2016 world champion Nico Rosberg paid a flying visit during the sunny morning session, completed 75 laps with a best time of 1 minute 19.705 seconds on ultra-soft tyres.

That was a significant step up from the 1:20.960 set by Ferrari's Kimi Raikkonen on soft tyres on Tuesday, which had ranked as the fastest of the opening 2017 test.

It also offered further evidence of how much quicker the new, wider cars are with their more aggressive aerodynamics and fatter tyres.

The fastest time in 2016 Barcelona testing was 1:22.765. Bottas's time would have put his car on pole position for every race at the Spanish circuit since 2007, when the layout was changed.

The 1.6-litre V6 power units were introduced in 2014.

Australian Daniel Riccardo was second fastest for Red Bull, 1.448 seconds slower. Renault's Jolyon Palmer -- who had an early spin into the gravel that brought out red flags -- was third with a best lap set on softs.

Ferrari's Sebastian Vettel completed more than a race distance before lunchtime, doing 69 laps on medium tyres, and was fourth fastest.

Canadian rookie Lance Stroll got back into the Williams after an early spin on Tuesday damaged the car and ended his first day of testing, and completed 56 laps. The 18-year-old also ended up back in the gravel.

Honda-powered McLaren continued to struggle, with double world champion Fernando Alonso managing only 28 laps and more than four seconds off the pace. The team said their main focus was gathering data.

Toro Rosso and Haas both spent long spells in the garage, with the former's Spanish driver Carlos Sainz doing just one installation lap in the morning.